If you're looking to buy an Apple Watch Series 4, the biggest changes include a new FDA approved ECG feature and fall detection, but hopefully you won't have to use those two features on a daily basis. What you may use regularly are its fitness features.

The Apple Watch Series 4 has added better training tools for athletes, automatic workout detection and competitions to motivate you to close those activity rings.

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The Apple Watch Series 4 delivers on its fitness promises

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But these are mostly software features you can get on its cheaper predecessors (Series 1 and up) with the WatchOS 5 update. The cheapest 40mm Series 4 model with the aluminum frame and no LTE starts at $400, $120 more than the Series 3. (For international pricing, see our Apple Watch Series 4 review.)

After testing out the Apple Watch Series 4 alongside the Series 3 for about three weeks, I noticed two major advantages over its predecessors when it comes to working out: screen size and battery life.

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Bigger, bolder, better

The first thing I noticed when I took the Apple Watch Series 4 for a run was how much easier it was to read the numbers. It's one of those things you never notice until it's improved upon, and all of a sudden I found it easier to track my heart rate and check my pace and distance, without having to squint at the screen and risk a stumble.

The Series 4 has undergone a complete redesign from the previous Apple Watch models. The screen has increased from 38mm to 40mm (for the smaller version) and 42mm to 44mm (for the larger). Plus the bezels are slimmer, so you're getting a lot more usable screen space on your wrist despite the body of the watch not being that much bigger. This means all your data (including activity stats) appears bigger and bolder on the Series 4, which makes a difference when you're in motion.

The Apple Watch Series 4 also has a slightly slimmer body than its predecessor, and I found the little sensor belly on the underside feels more comfortable when it's strapped on tight around my wrist.

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Battery life

But where the Apple Watch Series 4 really pulls ahead is in battery life. Apple claims it can last up to 6 hours of continuous outdoor activity tracking, compared with the roughly 4 hours you'd get on the Series 3. It's still much less than other specialized trackers on the market, such as those from Garmin and Polar, but it's a big boost for distance athletes considering an Apple Watch.

During the first week of testing, I didn't notice much of a difference in battery life between the Apple Watch 4 and 3. I'd finish the day with about 40 percent battery life left on the Watch 4 after a normal day of use and maybe an hour's worth of activity tracking thrown in there. Bottom line: I still had to charge it on a daily basis, so I was skeptical about the 6-hour claim.

To test it out, I took both the Apple Watch Series 4 and Series 3 on a 6-hour hike up Mount Tamalpais, about an hour from San Francisco.

Both watches were at 100 percent when I got out of the car at the trailhead parking lot near Stinson Beach, California. Before I began, I put both on airplane mode at full brightness, with the Series 4 on my left wrist and the Series 3 on my right, and started a hiking session.

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The hike up from Stinson Beach to the top of the mountain is pretty steep. You start out among the redwoods, traverse up a stream with a few waterfalls along the way and there's even a point where you have to climb up a ladder to continue. So while my pace may not have been very fast, I was continuously using the GPS, altimeter and heart rate.

At the hour mark I paused to check the battery levels and take a few shots of the screens: The Series 3 was at 75 percent and the Series 4 was at 87 percent.

Three hours in, I reached the summit and chomped on a sandwich as I walked (stopping was not an option for my test). The Series 3 was at 25 percent while the Series 4 was going strong at 65 percent. At that point I began to worry that I'd have to stay on the mountain past sunset.

I knew the way down would be faster, so I went on a few other trails at the top to kill some time before I started my descent. Exactly 4 hours in, I got a low-battery alert on the Series 3. I dismissed it and continued, and then saw all my progress disappear from the screen as the older Watch switched to Power Reserve. Meanwhile the Series 4 was still going strong with about 45 percent battery.

At that point it had already won, but I pressed on to see how long it would last.

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Six hours and 20 minutes, 12.75 miles and 2,570 feet of elevation later, I finally decided to call it a day and stop the hike. I still had about 5 percent battery left on the Apple Watch Series 4. After turning off activity tracking, I had enough power to get me back to the car and drive an hour back home before it went into Power Reserve.

Needless to say, Apple delivered on the 6-hour activity claim, and then some. Sure I didn't have cell signal turned on and I wasn't playing music, but it was at max brightness and it definitely out-hiked the Series 3 by at least 2 full hours. This means the Series 4 can probably get you through a full marathon.

And while the battery boost is mostly for activity tracking, it may be worth the upgrade for athletes.

But if you're not planning on signing up for any marathons or 6-hour hikes, many of the fitness upgrades are still available on older models.

WatchOS 5 makes the Apple Watch a better workout buddy

With the update to WatchOS 5, the Apple Watch can now detect when you're working out, regardless of whether or not you remembered to give it a heads-up. Within a few minutes of starting you'll get an alert and vibration prompting you to begin tracking and it will continue to nudge you until you accept or dismiss. If you accept halfway through your session, you'll get retroactive credit for anything you did up until you got the alert.

And it knows what kind of activity you're doing. I didn't try all of these, but it can theoretically distinguish between running (indoor or outdoor), walking (indoor or outdoor), swimming (open or pool), elliptical and rowing machine, though there's no cycling yet. It also knows when you've finished a workout, and reminds you to end your session with a nudge and a notification regardless of what kind of exercise you're doing.

This is a feature that Fitbit devices have had for a while now, but it's nice to finally get it on the Apple Watch.

My only complaint was that it kept wanting to record my walk to work as a run. The first couple of times I gave it a pass, because I was legitimately running, but I figured after a week of dismissing it at the same time it would figure out that I wasn't really running, just walking fast and sprinting between stoplights.

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Yoga tracking

As well as hiking, WatchOS 5 added yoga to the roster of activities you can select on the Watch. Granted you could always track under the Others category and then label accordingly in the Activity app, but having a designated category means the tracking algorithm is customized for that specific sport.

Activities tracked in the Others category are tracked similarly to a brisk walk and I noticed it now records more calories burned during hikes (not just the 6-hour ones).

Training tools for runners

If you're a runner like me, you're also getting a new set of tools to help you reach your personal record. You can now keep track of cadence (steps per minute) and rolling mile pace (how fast you ran the previous mile) while you're out pounding the pavement. My favorite feature is the pace alert, which makes the watch vibrate when you dip below or go above your target pace. It's definitely kept me from slacking off on my usual 4-mile running route and I can really see it being beneficial for races.

You can also race against your friends. WatchOS 5 adds competitive streaks to the Sharing tab on the Activity app by allowing you to challenge your friends to a seven-day ring-closing competition.

You get one point for every percent of the ring you complete. The winner gets an award that's unique to your competition.

Let's be honest, if you share your activity with your friends, you were probably already getting a little competitive about it. I know I was. This is just a way for Apple to make it official and raise the stakes. It's definitely motivated me to step up my game and close my move rings more often than before, but I also get borderline obsessive about winning that virtual medal during those seven days.

The verdict

Like its predecessors, the Apple Watch Series 4 is a worthy fitness companion for all levels. The longer battery and better screen are great incentives for distance runners who were waiting to make the plunge to an Apple Watch or just looking to upgrade from an older model. But 6 hours of tracking is still miles behind other dedicated fitness watches that can go for days on a charge.

The biggest selling point of the Apple Watch is that it does so much more than just fitness. The added cell connectivity means you can comfortably leave your phone behind on a run and still have all the essentials: music, messages, calls or car service.

And with the Series 4, you have access to new health features like the ECG feature that lets users utilize a single-lead electrocardiogram on their wrist, and the fall detection feature that alerts emergency services and contacts when the user takes a fall and can't get up. Be sure to check out our review before making a final decision.

Originally published Oct. 11.Update, Decemeber 7: Added roll-out of ECG feature to the Apple Watch Series 4.